1. Field of the Invention
This invention relates to optical networks. More particularly, the invention relates to modules that can be used to construct bandwidth-management systems for optical networks. The modules provide gain, power (or gain) equalization, and dispersion compensation on a band-by-band basis, facilitating flexibility in network design and ease of network expansion.
2. Description of the Problem
Fiber optic communication systems, such as those currently used in telecommunication distribution networks, typically require amplification of the signal light to compensate for optical power losses which occur over long distances. This amplification also serves to compensate for losses due to splitting the signal light between different branches of the network. Modern fiber optic communication systems utilize optical amplification devices based on optical fibers which have a core doped with a rare earth, such as erbium. Such a device is commonly known an erbium-doped fiber amplifier (EDFA). These amplifiers are well known in the art. Typical EDFA systems amplify a signal light by passing the signal light through the doped optical fiber while simultaneously pumping the fiber with a relatively powerful laser having a wavelength approximately equal to the absorption wavelength of the rare earth ions. EDFA amplifiers are common in modern optical networks, and have made possible the operation of extremely long networks covering large geographic areas without resorting to electronic regeneration to compensate for losses.
Despite the above advances in optical amplification, long optical spans still suffer from distortion problems that must be corrected if the network is to operate properly. Distortion may arise from a number of sources. Optical fiber systems inherently exhibit a property called dispersion, a pulse-broadening mechanism that reduces the bandwidth of the system. Dispersion may be caused by modal dispersion, in which different modes of a multimode optical fiber propagate at different group velocities. A second type is chromatic dispersion, which results from a combination of material dispersion in the optical glass and geometric effects of the waveguide. Chromatic dispersion causes different spectral components of a signal to propagate at different velocities, inducing pulse spread in high-bit-rate systems.
Once consecutive pulses have spread out so that they are no longer distinguishable from one another, information is lost. Dispersion may be compensated for on a channel-by-channel basis. Various dispersion compensation mechanisms are known. A common dispersion compensation system consists of a length of dispersion compensating optical fiber connected to the system. This special fiber exhibits dispersion characteristics that cancel the dispersion characteristics of the network. U.S. Pat. No. 5,861,970 issued Jan. 19, 1999 provides a good discussion of dispersion in optical networks and is incorporated herein by reference.
Even with known methods of dispersion compensation, dispersion differences between channels lead to non-ideal compensation over all bands. These differences increase with the number of channels and the system length. Dispersion can exceed several thousand picoseconds per nanometer for long-haul systems. FIG. 1 illustrates a dispersion map, a plot of dispersion versus system length. The map of FIG. 1 illustrates approximate dispersion for terrestrial systems using large effective area fiber for 16 channels ranging from 1531 nanometers to 1559 nanometers. In this map, dispersion compensating fibers are used at approximately 80 kilometer intervals. Curve 101 approximates an average dispersion for wavelengths from 1553 to 1559 nanometers, curve 102 approximates an average dispersion for wavelengths of 1540 through 1543 nanometers, and curve 103 approximates dispersion for wavelengths in a range of 1531 to 1533 nanometers. The difference in dispersion compensation, xcex94D, for this band of frequencies in this relatively short system is approximately 750 picoseconds per nanometer (ps/nm).
FIG. 2 shows a similar dispersion map as shown in FIG. 1, but for submarine large effective area fiber. Again the dispersion is compensated for using standard single mode fibers every 80 kilometers. Curves 201, 202, and 203, represent average dispersion for the same wavelength ranges as those shown at 101, 102, and 103 in FIG. 1, discussed above. In this case, the difference in dispersion across the bandwidth of the system is approximately 900 ps/nm. As the spectral bandwidth of deployed systems increases, xcex94D will increase, and the required dispersion compensation will vary across the spectrum, making it more and more difficult to design optical networks which provide error-free communication.
Other problems with large optical networks result from inadequate gain equalization. With dense wavelength division multiplexing (DWDM), many channels or transmission signals independent of each other are sent over the same line or optical fiber by multiplexing within the domain of optical frequencies. The transmitted channels are distinguishable from each other because each of them is associated with a specific frequency or wavelength. In an optical network, the different channels must be substantially equivalent to each other in terms of signal level. However, doped fibers, as typically used in optical amplifiers, have an emission spectrum with a peak of limited width; the features vary depending on the glass system into which the dopant is introduced, as well as other factors. The accumulated wide-power variation among channels in DWDM systems can deteriorate the overall system performance significantly. FIG. 3 shows the impact of gain variation with system length for a submarine system, assuming a gain tilt of xe2x88x920.5 dB to +0.5 dB from short to long wavelengths. For the channels that experience a larger than nominal gain, shown at 301, the channel power grows with system length. As the system length increases, the channel power increases above the threshold, 303, for non-linear interaction of the channels in the fiber. Additionally, the channels that experience a smaller than nominal gain, 302, will lose power with system length. As the system length increases, the channel power will drop below the detection limit 304. For adequate signal recovery with an optical amplifier there should be sufficient optical signal at the input. Operating below the detection limit or above the linear behavior limit results in degradation of the signal and the information on that channel is not recoverable.
FIG. 4 shows a similar graph, but this time, for terrestrial optical systems. Curve 402 represents wavelengths decreasing in power and curve 401 represents wavelengths increasing in power. Operational limits are shown at 403 and 404. Filters are often used to provide gain equalization for optical systems to compensate for the effects described immediately above. Other gain equalization methods exist. Gain equalization is also often referred to as power equalization. U.S. Pat. No. 5,852,510, issued Dec. 22, 1998 provides a good discussion of gain equalization, and is incorporated herein by reference.
The interaction of the various requirements for amplification, dispersion, and gain equalization, as discussed above makes the design and configuration of DWDM optical fiber networks difficult and complex. Accommodating or compensating for one of these factors affects the others, and the effects are not always predictable across the network. The problem is especially acute when network expansion is required. As more modes or channels are added to the network, amplifiers, gain equalizers, and dispersion compensators which previously assured adequate performance, no longer work across the entire spectrum and entire network redesign becomes necessary. What is needed is a network design capability to provide for all of the above needs simultaneously on a channel-by-channel or band-by-band basis. Such a solution should ideally also allow for network expansion with little or no disturbance to the existing network topology.
The present invention solves the problems discussed above by providing intelligent, miniaturized, bandwidth-management modules (BMM""s) which subdivide the wide available spectrum into narrow band segments. Each individual BMM is designed to overcome loss, optimize dispersion and provide power (or gain) equalization for a few channels at a time. Multiple BMM""s can be concatenated together in an array (a bandwidth-management array or BMA) to provide a stepwise constant approximation to a broadband spectrum. Since amplification, dispersion compensation, and power equalization are all provided together for each set of channels, expanding a network to achieve greater bandwidth is easy, and relatively economical compared to prior art methods of network expansion. Additionally, networks built with the BMM""s of the present invention exhibit better power equalization and dispersion characteristics than networks built using traditional components.
Throughout this disclosure, we refer to the selection of channels handled by an individual bandwidth-management module as a xe2x80x9cbandxe2x80x9d of channels or wavelengths. We refer to the entire frequency range of channels handled by the network as the xe2x80x9cspectrumxe2x80x9d of channels or wavelengths. When we refer to xe2x80x9ccompensationxe2x80x9d we are referring to at least dispersion compensation, but in some cases both dispersion compensation and power equalization. We sometimes refer to controlled gain. By controlled gain we usually mean power equalization (or gain equalization) according to any method disclosed herein, regardless of the degree or method of power equalization, and regardless of whether it is channel-by-channel or band-by-band, or over time for a group of channels. We use the term gain and amplification interchangeably. Finally, we refer to a collection of BMM""s connected together with other components to manage the entire spectrum for a specific node within an optical network as a xe2x80x9cbandwidth-management system,xe2x80x9d and we refer to the BMM""s so connected as a xe2x80x9cconcatenatedxe2x80x9d bandwidth-management array (BMA).
According to the invention, a device is provided which exhibits controlled gain for a band of optical channels selected from a spectrum of optical channels in an optical network. The device includes an input block and an output block with an optical path in between. The input and output blocks include optical connectors and filters, as well as any other components necessary to direct the band of optical channels through the optical path while passing other optical channels within the spectrum to additional devices which can be connected without disturbing existing bandwidth-management modules. Each module also includes an amplifier connected in between the input and the output, and a compensation module that provides dispersion compensation specifically for the band of channels being handled by the device. The BMM further includes a digital control module which is programmed to monitor the average power in the band at the input and the output of the device. The control module can provide power or gain equalization by controlling one or more elements within the BMM. Power (or gain) equalization can be provided by controlling the laser current pumping the optical amplifier or by wavelength dependent loss elements such as attenuators and etalon cavities. In the preferred embodiment, dispersion compensation for the device is provided by a set of chirped optical fiber xe2x80x9cBraggxe2x80x9d gratings connected to an optical circulator. The gratings can either be connected in series or in parallel. Gain equalization can also be provided by varying the channel-specific reflectivity of the gratings.
Bandwidth-management systems for an optical network can be easily assembled by making connections to one or more bandwidth-management arrays, each comprising a number of concatenated bandwidth-management modules. Such bandwidth-management systems can be used to form any part of the network through the inclusion of multiplexers and demultiplexers as necessary. The bandwidth-management systems can be used for transmitters, receivers, or as line amplifiers and repeaters. Each BMM within the BMS is responsible for a different band of channels selected from the spectrum of optical channels handled by the network. Preferably, a master controller is connected to the digital control modules for each of the BMM""s to control the operation of the system. Crosstalk reduction within the optical network can be achieved by staggering the band selection from one BMA to another BMA so that adjacent bandwidth-management arrays group the same spectrum of channels into different bands of channels.
Each module of the invention is controlled by a digital control module which provides intelligence so that optimal characteristics are continuously maintained. The control module includes hardware to sample and compare the frequency dependent input power for the band of optical channels to which the module is dedicated, and sample and compare the frequency dependent output power for the same band. The pump laser power and current can also be monitored. The controller can continually adjust the module to keep an output property constant relative to the input power, a selectable fixed value, or a value provided by a master controller. In the preferred embodiment, it can also control the pump laser for constant power or constant drive current. We refer to the controller as xe2x80x9ccontrollingxe2x80x9d the module to provide controlled gain regardless of which components it is connected to within the module and how it is manipulating the components. It may provide controlled gain by controlling the amplifier, or by controlling loss elements, or a combination. The digital control module, the microcode which operates it, and the various amplifier, input, output, and compensation components all work together to provide the means to implement the modules of the invention.